The Spam Act specifically prohibits automatic harvesting of emails and I expect it's the same in the UK and USA, but I'm no authority on laws in either of those countries.
Rather than send spam to addresses harvested, you are better off working with opt-ins who are interested in your product or service and elect to receive information. AFter all, most people, if they are like me, never read spam and never reply to it.
I have an email harvesting software program too, but I no longer use it to harvest from the Internet, simply to maintain my existing lists.